You first carefully measure your lawn and enter its dimensions into an application on your PC. (You would also have to identify permanent obstacles as well such as that garden gnome who keeps staring at you.) The software then enables you to write/design the pattern via several methods (mouse, typing,
pre-existing shapes, etc). Then an RF tranmitter sends the data to a receiver on the mower. You would have to start the mower, but then the mower would do the rest.

Write your name in your lawn. In script. Write "I <heart> you" for your wife on her birthday. How about your favourite team crest/logo? Perhaps a large corporation will pay to have their logo mowed into your turf as a publicity stunt? Or perhaps you're more artistic and would like to generate your own patterns? Fractals? Simple (and nearly perfect I might add) diagonal lines?

Electromagnets at each wheel can raise/lower the cutting deck, thereby enabling a 3-D effect.

"Normal cutting" mode will take care of the dual drudgery of lawn mowing and "erasing" your artwork as something like an etch-a-sketch.

Of course you'd have to keep the lawn well watered and fertilised for the full effect to be realized. And you may have to forego bagging and let the cuttings lie where they may until you rake them up.

I love this idea. As for programming it, the easiest way would be to manually walk it around the perimeter of your yard. The mower/computer would store that date and send it to your computer, creating a map that you could draw on top of.